1. Field of the Invention
The present invention generally relates to a thermosetting resin composition, suitable for use in a process for preparing a smooth board. More particularly, the present invention relates to a thermosetting resin composition suitable for an undercoat of a printed wiring board without unevenness on its surface.
2. Discussion of Background
A printed wiring board is a board on which a conductor circuit is formed on one or both sides of an insulating substrate. It may be further coated with a protective layer of solder-resist as a topcoat.
However, a conventional printed wiring board coated with a topcoat has a defect in that hollows (6) form in the surface of the topcoat (2) as shown in FIG. 7. As a result, there have been problems in that reliable connections cannot be obtained when electronic parts are mounted and a fine particles of solder enter the hollows (6), to causing short-circuits.
The present inventors have studied the above-described problems and have concluded that the hollows (6) on the surface of the topcoat (2) are brought about by the recesses (7) on the surface of the insulating substrate (4) formed by the presence of the conductor circuit (3) as shown in FIG. 8. Accordingly, in order to obtain a smooth topcoat without unevenness, a thermosetting resin is filled into the recesses (7) between the conductor circuits and cured to form an undercoat (1) and then the undercoat (1) is polished to form a smooth surface to which the topcoat (2) is laminated as shown in FIG. 1.
Various kinds of thermosetting resin compositions have been proposed as a surface coating agent for a printed wiring board. For example, Unexamined Japanese Patent Publication (Kokai) No. 63-81187 and Unexamined Japanese Patent Publication (Kokai) No. 63-154780 describe a thermosetting resin composition comprising an epoxymethacrylate resin, a copolymerizable crosslinking agent such as acrylic ester and the like, a radical polymerization initiator, a liquid epoxy resin and a curing agent.
However, when the resin composition described in the above-described publications is applied to a printed wiring board as the above-described undercoat, air bubbles often remain in the resin coated. As a result, air bubbles remain in the hardened film (undercoat) and so the properties of the printed wiring board, such as heat resistance, humidity resistance and the like suffer. Therefore, the resin composition described in the above-described publications is not suitable as an undercoat.
Unexamined Japanese Patent Publication (Kokai) No. 8-162573 describes a thermosetting resin composition comprising a novolak-type epoxy resin, a curing agent, a crystallizable epoxy resin, and a curing accelerator such as imidazole and the like.
However, when the resin composition described in the above-described publication is applied to a printed wiring board as an undercoat and is cured, an extremely hard undercoat is formed. Therefore, the polishing of the surface is very hard to carry out, and fine polishing can not be carried out. As a result, a highly smooth surface can be hardly obtained. Therefore, the resin composition described in the above-described publication is also unsuitable as an undercoat.
An object of the present invention is to provide a thermosetting resin composition which does not leave air bubbles in a cured film and which forms a cured film whose surface can be easily polished.
According to the present invention, there is provided a thermosetting resin composition comprising (I) an adduct of epoxy resin with unsaturated aliphatic acid, (II) a (meth) acrylate, (III) a radical polymerization initiator, (IV) a crystallizable epoxy resin, and (V) a latent curing agent.
Thus, a smooth board can be prepared by applying the above-described thermosetting resin composition to the recess in the surface of the substrate, carrying out a primary curing at a low temperature, polishing the surface and then carrying out a secondary curing at a high temperature.
In a preferred use, the present invention provides a process for preparing a smooth printed wiring board which comprises applying the above-described thermosetting resin composition to the recess in the surface of a printed wiring board, carrying out a primary curing at a low temperature, polishing the surface and then carrying out a secondary curing at a high temperature. This provides a smooth printed wiring board. Another process comprises preparing a smooth multi-layer printed wiring board by applying the above-described thermosetting resin composition to a via on the surface of the multi-layer printed wiring board, carrying out a primary curing at a low temperature, polishing the surface to form a conductor circuit, coating with an insulating layer and /or protective layer, and then carrying out a secondary curing at a high temperature. This also provides a smooth, multi-layer printed wiring board.